A Closer Look: The Role of Water Vapor

Isn’t water vapor the primary greenhouse gas?

Yes, water vapor is the primary greenhouse gas. In fact, it contributes between 36% and 66% of the Earth’s greenhouse effect that keeps Earth’s temperature hospitable, depending on location and cloud cover. Odds are you’ve noticed the impact of water vapor on temperature without even realizing it. If you’ve ever spent an evening in a very humid location, where there is a lot of water vapor in the air, you’ve noticed that it still remains quite warm after the sun goes down. This is because the water vapor is helping to keep that heat close to the Earth’s surface, preventing its escape to space. If you’ve ever spent an evening in a very arid location, you’ve noticed just the opposite, that evenings and nights can get quite cold since there is very little water vapor to retain the daytime heat.

While powerful in its effects, water vapor only remains in the atmosphere for a very short period of time, generally only a few days. It enters the atmosphere through evaporation, condenses into clouds, and then returns to the surface in the form of precipitation. As a result, the percentage of the atmosphere that is made up of water vapor varies greatly, between 0 and 4%, based on geographic location, time of year, and so on. The percentage also varies based on the distance from the Earth’s surface. The higher you go into the atmosphere, the drier it gets. The troposphere, which only extends about 10 miles above the surface, contains about 75% of the Earth’s atmospheric mass and is where the Earth’s weather occurs.

So why is CO2, which makes up such a small concentration in the atmosphere, so important? Unlike water vapor, CO2 can remain in the atmosphere for a very long period of time, upwards of 100 years. CO2 gas is also well mixed throughout the atmosphere, which is to say that you can measure the CO2 concentration at different locations and at different heights from the Earth’s surface, and you will get very close to the same result. This is why CO2 measurements can be performed at a single location like Mauna Loa in Hawaii. So, while not as powerful a greenhouse gas as water vapor, the effects of CO2 are universal and long term. And this constant effect can influence the role of water vapor.

As CO2 warms the air, the air can hold more water vapor, which in turn warms the air further. So, CO2 and water vapor can work hand-in-hand to drive temperatures higher, with CO2 driving the effect and water vapor providing a positive feedback loop, enhancing the effect of the CO2.

Increased water vapor in the air can also promote the formation of more clouds. Depending on their type, clouds can either hold heat in, further enhancing the greenhouse effect, or reflect heat back into space, working in opposition to CO2 and driving temperatures lower. This dual role of clouds is one of the most complex areas of study related to climate change.